W. A. E. Ussher — TJie Culm of Devonshire. 



15 



The junction between the Lower Culm Measures and Upper 

 Devonian slates of North Devon is very seldom visible, but there are 

 cases, as in the railway cutting near Dulverton Station, where we 

 pass from shales distinctly belonging to the Culm Measures to shales 

 lithologically indistinguishable from them in which Petraia ceJtica 

 was obtained. 



Phillips says, p. 189, "The exact circumstances of the passage 

 from the upper part of the Pilton group to the lower part of the 

 Carbonaceous group . . . are scarcely anywhere completely seen in 

 North Devon. There is usually a longitudinal valley on the line of 

 junction which obscures the phenomena. Fremington Pill is per- 

 haps the best locality for observation of this passage." As to the 

 junction between Culm Measures and Devonian in South Devon, it is 

 one of the questions of the future. Phillips points out appearances 

 of unconformity between the Petherwin beds (Upper Devonian) and 

 Lower Culm Measures at Landlake, south of Launceston, op. cit. p. 196, 

 and I do not think that the comparative absence of the Upper Devo- 

 nian strata has been accounted for in South Devon. On the broad 

 questions of correlation many plausible suggestions have been made ; 

 but if the Pilton beds are unquestionably representative of the 

 Lower Carboniferous Slate of Ireland, what becomes of that repre- 

 sentation in South Devon ? Is it possible that the Upper Devonian 

 Slate series is in South Devon represented by Lower Culm Measures, 

 through that Division attaining a much greater thickness in the 

 southern area ? It may be that the greater extension of Lower 

 Culm Measures in the southern area is due to low dips or more 

 constant repetition by faults and flexures, see F. Eutley " On the 

 Schistose Volcanic Kocks of Dartmoor," Q. J. G. S. vol. xxxvi. p. 

 287, 1880, and Harvey B. Holl, Q. J. G. S., vol. xxiv. p. 407, 1868. 



Middle Culm Measures. — These rocks occupy the coast from West- 

 ward Ho to Portledge Mouth. The breadth of their outcrop from 

 Northam to Wear Giffard in the longitude of Bideford is about four 

 miles ; their southern outcrop from the Triassic valley of Crediton 

 northward embracing Morchard Bishop, Iddesleigh, Meeth and 

 Petrockstow, is from 4 to 5 miles in breadth. 



Fig. 1. 



Fig. 2. 



The Middle Culm Measures are a very variable series, their special 

 characteristics are rather coarse sandstones, which, near Zeal Mona- 

 chorum and other places in their southern outcrop, occur in bfds 

 about five feet thick, and are associated with marly splitting shales 



